Ultrasonic welding has long been used to join together two thermoplastic pieces. Prior to the present invention, it had been thought that materials having different properties, such as a rigid piece and a flexible piece, were generally not suitable for being ultrasonically welded together. It had also been thought that it was difficult to ultrasonically weld plasticized polyvinyl chloride with good results, especially to another type of material.
Energy directors have been used to improve ultrasonic welding. These prior art energy directors typically have a single peak, which is sometimes roughened for particular applications.